This invention relates to the production of articles from atomized metals, and, more particularly, to the formation and control of a spray of atomized metal droplets and apparatus for producing articles in this manner.
In a common method of forming metallic articles, a metal alloy is melted and then cast into a mold. The mold cavity may have the shape of the final article, producing a cast article. Alternatively, the mold cavity may have an intermediate shape, and the resulting billet or ingot is further processed to produce a wrought final article. In either case, the solidification rate of the metal varies over wide ranges and produces wide variations in structure, particularly where the article is large in size. Moreover, the internal metallurgical microstructure of the article often has irregularities that interfere with its use. Such inhomogenieties such as chemical segregation and variations in grain size, and irregularities such as voids, porosity, and non-metallic inclusions, may persist after considerable efforts to remove them.
Articles may also be produced through the use of melt atomization techniques. In this approach, metal is melted and atomized into small droplets. The droplets may be permitted to solidify in that form as powder, and the powder is formed into the article. Although this approach would seem to be rather indirect, it has important advantages in achieving higher and more uniform solidification rates of the structure, more regular metallurgical microstructures, and reduced waste as compared with machined products. A related technique is to deposit the spray of molten droplets onto a form or substrate, gradually building up the mass of metal until the article is formed. The article may be of the final form required, or a billet that is further processed to the final form. This approach is used to achieve rapidly solidified structures with homogeneous metallurgical microstructures, and which may require little subsequent processing to the final form.
Although the metal spraying approach substantially improves the structure of the article, the process may be improved by achieving better control of the metal spray. For example, the characteristics of the final article may depend upon the way in which the spray of molten metal droplets is formed. Or, in the approach where the spray of articles is deposited upon a substrate, even when a relatively regular shape such as a cylindrical billet is formed by metal sprayed onto an end of the billet, the microstructure near the outer periphery of the billet is usually finer in scale than that near the centerline of the billet. The outer periphery cools faster than does the centerline, which may result in difficulty in adhering the sprayed particles to the areas on the periphery, thereby reducing process yield, and may result in centerline porosity, cracking, and distortion. Additionally, some molten materials, including the reactive metals such as titanium, are extremely reactive with the ceramic materials necessary for producing metallic and metallic-based products by conventional techniques. Processes for the production of such materials, for example spray atomization to produce metal droplets and powder (upon solidification) are uneconomical due to the short production runs achievable. Alternatively, with longer runs, the contamination levels become unacceptable from a mechanical properties standpoint because properties such as low cycle fatigue are strongly influenced by foreign particle contamination of the melt, in particularly due to contamination from non-metallic inclusions.
Further, the nozzle may be linked to a cold hearth melting system wherein the molten material only contacts a skull of the same composition as the melt, precluding contamination from the melt containment vessels or flow control nozzle. Coupling a semi-continuous feed system to a cold hearth melting system and the invention disclosed herein enables extended economical production of a spray of atomized metal droplets. Such systems are described in U.S. Pat. No. 5,120,352 and concurrently filed, U.S. Pat. No. 5,171,358, incorporated herein by reference.
There is therefore a need for an improved technique for producing a spray of molten metal and depositing sprayed metal particles onto substrates, to achieve more regular macrostructures and microstructures. The present invention fulfills this need, and further provides related advantages.